This invention relates generally to the output power control of power supplies, and more particularly to output power control of multiple power supplies coupled in parallel to allow load-sharing therebetween.
As more and more segments of the business environment enter the information age, more and more computers and computing power are required. As business move from the old to the new economy their reliance on the processing, transference, and storage of digital information is becoming a more and more critical aspect of their overall business strategy. While in the past, computer crashes were seen as a mere nuisance, the loss of computing power and business data may well devastate a business""s ability to survive in today""s new economy. As such, the need for reliable, uninterruptible electric power to maintain the operational status of the computing equipment and the integrity of the digital data continues to rise.
To meet these requirements, uninterruptible power supplies (UPS) have been developed. These UPSs utilize a bank of electric storage batteries and solid state conversion equipment in association with the utility line voltage to provide continuous electric power to a businesses computer systems in the event of a loss or deviation of power quality from the utility. The number of batteries contained within an UPS is dependent upon the businesses length of time that it needs to operate in the event of a utility power system failure. Likewise, the number of power modules included in a modular UPS is dependent on the overall total system load required to be supplied thereby. While these new modular UPSs allow a user to increase the power that may be supplied as their needs increases by simply adding additional modules, some businesses may find it necessary to purchase additional UPSs to supply their total power need.
If the outputs from the separate UPSs are electrically isolated from one another, each may supply power to its connected loads without concern for the output power being supplied by other UPSs. However, one of the advantages of the modular UPSs is that the combined power from each of the modules is available to supply the entire system load. This eliminates the need to carefully monitor how much load is connected to each individual unit, since the total system load will be shared by the various power modules. Within a modular UPS this paralleling control is easily accomplished, typically through integrated master/slave controllers contained in the UPS housing. However, such paralleling control for systems that have multiple distributed UPSs (as opposed to a single, modular UPS) present different problems that must be addressed.
As illustrated in FIG. 1, unlike the identical power modules within a modular UPS that may use simple master/slave controllers, the different line impedances Z1, Z2 . . . Zn that couple the distributed supplies 22, 24, 26 to the various loads 28, 30 makes this simplistic type of control unfeasible. Further, if the system includes supplies of different rating or from different manufactures, the different source impedances also renders this type of control unable to properly maintain parallel load sharing between the sources. Additionally, since there is no standard for this type of paralleled system, different power supply manufactures typically not include the ability to act in a master or a slave supply role anyway. Even if such control were able to adequately load share while operating in parallel, multiple discreet control wires would be required between the supplies 22, 24, 26 to carry the master waveform information. This would greatly increase the cost and complexity of this system, and would reduce its overall reliability.
Other multiple-source paralleling techniques also require that multiple control or sense wires be utilized. One common technique is to place current and voltage sensors at the source terminals and at a point of regulation near the load connections. A complex interconnection of control wires is then used to determine a difference-from-average (DFA) current that is being supplied by each individual supply. This DFA current represents a load sharing unbalance, and is used by the individual supply units to adjust their output regulation to drive the DFA current to zero, thus supplying equal load on each unit.
In addition to the increased cost and complexity of such an arrangement, this type of system does not account for the potential different ability of any one source to supply a given load. That is, each unit will be driven to supply equal load, even if its capacity is less than another unit (due to the use of different rated units in the parallel system). This may cause smaller rated units to fail in situations where the overall paralleled load is well below the combined capacity of the overall system. For example, if a supply rated at 1 kVA and one rated at 5 kVA are paralleled, the total system capacity is 6 kVA. However, if the parallel control system attempts to maintain equal load sharing between supplies, the 1 kVA rated supply will likely fail at connected loads above 2 kVA since its equal share of the load will exceed its rated capacity.
There exists, therefore, a need in the art for a system and method to allow multiple supplies, particularly UPSs, to be paralleled without requiring interconnection of control wiring between units, and to allow load sharing in proportion to the individual supply""s power rating.
To overcome the above described and other problems existing in the art, the system and method of the present invention provide an adaptive feedback control system that enables connectionless parallel operation of two or more power supplies. The power supplies can be any electronic devices with output control loops, such as DC/AC Inverters, UPS (Uninterruptible Power Supply) units, etc. The control system and method of the invention are incorporated within the output controller for each supply, and operates without the requirement of any communication of control or load parameters from any other supply or its controller. Further, the system and method of the invention do not require any difference from average (DFA) current information or any remote point-of-regulation voltage data to maintain proper parallel operation. Unlike prior paralleling and load sharing systems, the system and method of the invention maintains proper load sharing in proportion to the capacity of the individual source, as opposed to as an equal division of the load between the paralleled sources without regard to source capacity.
This new paralleling power supply technology is applied to the output control loop of each power supply. An adaptive output voltage reference, Vref, which is the sum of a desired output voltage waveform and a compensator is used to control the output waveform. The compensator utilizes two feedback loops, one for its supply""s output current and another for its supply""s output voltage, to adaptively modify the normal reference control signal, Vref. In this way the system and method of the invention balances currents among and shares load with the other power supply units that are supplying power in the same power supply system. With this dynamic control scheme, a bank of power supplies, such as UPS units, inverters, etc. having different power capabilities, different impedances, different phase controllers, and different harmonic profiles can be paralleled together to supply power to the connected loads without any inter-connection or communication among, these power supplies.
In a preferred embodiment of the invention, the method of controlling an electrical power source, which is in parallel operation with at least one other electrical power source to supply an electrical load, comprises the steps of generating a desired reference voltage signal, sensing an output voltage and an output current generated by the electrical power source, and compensating the desired reference voltage signal with only the sensed output voltage and the sensed output current to maintain proper division of the electrical load on a per unit basis. Preferably, the step of compensating comprises the steps of calculating a difference between a magnitude of the reference voltage signal and the sensed output voltage, integrating the difference, adding a constant selected based on a power capacity of the electrical power source, multiplying this result by the reference voltage signal, and subtracting the sensed output current to develop a compensation signal. This compensation signal is then added to the reference voltage signal.
In an alternate preferred embodiment of the invention, an electrical source is presented that is capable of operating in parallel with other electrical power sources to supply a connected electrical load. This electrical source comprises an output voltage generator producing an output voltage and an output current. It also includes a controller that calculates a reference control signal in a connectionless manner with the other electrical power sources. The reference control signal is used by the output voltage generator to control the output such that the electrical source supplies an amount of the connected electrical load in proportion to a total power capacity of the electrical source itself. Preferably, the controller calculates the reference control signal by sensing only the output voltage and the output current. The controller includes a dynamic feedback adaptive control system that utilizes the output voltage and the output current to compensate a desired voltage reference signal to achieve the reference control signal.
This dynamic feedback adaptive control system includes a slow voltage control loop and a fast current control loop. The slow voltage control loop integrates a difference between the output voltage and the desired voltage reference signal, and the fast current control loop provides proportional compensation that is inversely proportional to the output current. In one embodiment, the slow voltage control loop includes the addition of a constant related to the total power capability of the electrical source. Preferably, the fast current control loop operates to compensate the reference signal due to load unbalances, and the slow voltage control loop operates to minimize compensation of the desired voltage reference signal by the fast current control loop for changes resulting from changes in electrical load.
In yet a further preferred embodiment of the invention, a dynamic adaptive feedback controller for an electrical power source that is capable of supplying electric power in parallel with other power sources to a connected electrical load is presented. This controller comprises a desired reference waveform generator, and an output voltage sense and an output current sense coupled to the output of the electrical power source. This controller utilizes a slow voltage feedback loop and a fast current feedback loop to maintain proper load division on a per unit basis in a connectionless manner with the other power sources. Preferably, the controller calculates a difference between the magnitude of the reference voltage signal and the sensed output voltage, integrates this difference, adds a constant selected based on a power capacity of the electrical power source, multiplies by the desired reference voltage signal, subtracts the sensed output current to develop a compensation signal, and adds the compensation signal to the reference voltage signal to control the output of the electrical power source.
Other objects and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings.